Transforming growth factor-beta (TGF-b) is a multifunctional cytokine that regulates angiogenesis by signaling through the type I TGF-b receptor, ALK-5, and two endothelial cell specific TGF-b receptors, ALK-1 and endoglin. ALK-1 and endoglin form a complex together and are essential for endothelial cell function as demonstrated by 1) their mutation in the human vascular disease, hereditary hemorrhagic telangiectasia (HHT), and 2) the lethal phenotype due to angiogenesis defects observed when each gene is targeted for deletion in mice. However, mechanisms regulating the balance of TGF-b signaling through ALK-1 and ALK-5 in endothelial cells and the role of endoglin in regulating this balance are poorly understood. Based on preliminary studies we will test the hypothesis that the balance of TGF-b signaling through ALK-1 and ALK-5 to control endothelial cell proliferation and migration is regulated by 1) ALK-1 binding to and phosphorylating the nuclear receptor, LXRbeta (LXR-b), to inhibit ALK-1 signaling, 2) endoglin binding to the cytoplasmic domain of ALK-1 to activate ALK-1 signaling and ALK-1 phosphorylating endoglin to inhibit ALK-5 signaling and 3) regulation of endoglin expression by binding the PDZ domain containing protein, GIPC. Specific Aim I: The sites of interaction between LXR-b and ALK-1 and the sites of LXR-b phosphorylation will be mapped and mutants lacking these functions used to establish the mechanism by which LXR-b inhibits ALK-1 signaling. Specific Aim II: The sites of interaction between endoglin and ALK-1 and the sites of endoglin phosphorylation will be mapped and mutants lacking these functions will be used to establish the mechanism by which endoglin activates ALK-1 signaling. Specific Aim III: The sites of interaction between endoglin and TbetaRII will be mapped and mutants lacking this function will be used to establish the mechanism by which ALK-1 inhibits ALK-5 signaling. Specific Aim IV: The effect of GIPC expression on endoglin functions will be explored to define the mechanism by which GIPC decreases endoglin expression to regulate ALK-1 and ALK-5 signaling. The effects of these pathways on endothelial cell proliferation; migration and angiogenesis assays will also be investigated. The results from these studies will aid in the design of treatments for patients with HHT, other vascular disorders and angiogenesis-dependent solid tumors.